Extending the low-temperature operation of sodium metal batteries combining linear and cyclic ether-based electrolyte solutions

Nonaqueous sodium-based batteries are ideal candidates for the next generation of electrochemical energy storage devices. However, despite the promising performance at ambient temperature, their low-temperature (e.g., < 0 °C) operation is detrimentally affected by the increase in the electrolyte resistance and solid electrolyte interphase (SEI) instability. Here, to circumvent these issues, we propose specific electrolyte formulations comprising linear and cyclic ether-based solvents and sodium trifluoromethanesulfonate salt that are thermally stable down to −150 °C and enable the formation of a stable SEI at low temperatures. When tested in the Na||Na coin cell configuration, the low-temperature electrolytes enable long-term cycling down to −80 °C. Via ex situ physicochemical (e.g., X-ray photoelectron spectroscopy, cryogenic transmission electron microscopy and atomic force microscopy) electrode measurements and density functional theory calculations, we investigate the mechanisms responsible for efficient low-temperature electrochemical performance. We also report the assembly and testing between −20 °C and −60 °C of full Na||Na3V2(PO4)3 coin cells. The cell tested at −40 °C shows an initial discharge capacity of 68 mAh g−1 with a capacity retention of approximately 94% after 100 cycles at 22 mA g−1.

in eight different electrolyte solutions at a current density of 0.5 mA cm −2 and a capacity of 0.5 mAh cm −2 . Note that performance (overpotential and cycle life) of the Na metal electrode in these eight electrolytes exhibits the following decreasing trend: NaOTf-DEGDME > NaFSI-DME > NaFSI-DEGDME > NaClO4-DME > NaClO4-DEGDME > NaTFSI-DOL > NaTFSI-DEGDME > NaTFSI-DME. with a capacity of 0.5 mAh cm −2 . Note that the 1 M NaTFSI-DME and NaTFSI-DEGDME electrolyte solutions showed early failure before the 50th cycle, so their curves are not displayed here.
Note that the cell with 1 M NaTFSI-DME failed at the 22nd cycle and the cell with 1 M NaTFSI-DEGDME failed at the 16th cycle. The cells with 1 M NaFSI-DEGDME and NaClO4-DME were stopped after 50 cycles. Insets: corresponding cross-sectional SEM images.

Supplementary
For Cl 2p XPS peak-fitting, the 2p3/2 to 2p1/2 area ratio is fixed at 2:1 and the doublet separation is 1.60 eV. In the Cl 2p profile, the peak at 198.4 eV (based on 2p3/2) is assigned to Cland the peaks at 208.5 eV and 206.3 eV can be ascribed to ClO4 -(based on 2p3/2) and ClO3 -(based on 2p3/2) 10-13 , respectively.
(symmetric Na||Na cells) in 1 M NaClO4-DME electrolyte at +20˚C at a current density of 0.    Figure 2a and 2b using the equivalent electrical circuits (insets in Supplementary Figure 2a and 2b). Note: "appear" indicates that a component is not detected on the surface but observed in the bulk;

Value
"disappear" indicates that a component is shown on the surface but not in the bulk; "N/A" indicates that same components are observed on the surface and in the bulk. impedance data of stainless steel||stainless steel symmetric cells containing 0.5 M NaOTf-DEGDME/DOL (5:5) solution at a range of temperatures shown in Supplementary Figure 27b using the equivalent electrical circuit (inset in Supplementary Figure 27b). (1) There is distinct difference in the Na metal electrode surface morphology associated with NaClO4 salt at +20°C and −20°C. For example, a severely damaged surface can be observed for NaClO4-DEGDME at +20°C, while a relatively smooth surface (with pores identified from cross-section view) can be seen at −20°C. Similar trend can be observed for the case of NaClO4-DME. This may be due to the significantly reduced reaction between the NaClO4 salt and Na metal with decreasing temperature. This is also consistent with the electrochemical behavior investigation, in which the electrolyte with NaClO4 salt operates much better at low-temperature compared to room-temperature condition.

Re (Ohm
(2) For the cases with NaFSI and NaTFSI salts, the electrolytes with NaFSI generally lead to less fractured and porous Na metal electrode surfaces compared to the ones with NaTFSI.
Even though LiTFSI is reported as a frequently used salt for Li metal anode [20][21][22][23] , NaTFSI is shown to be a less promising candidate for Na metal anode. Figures 13 to 21 present the XPS spectra, depth profiles and analyses on the chemical composition of the SEI formed in NaClO4-DME, NaFSI-DME, and NaTFSI-DEGDME at +20°C and −20°C (four out of the eight electrolyte candidates were surveyed for the XPS analysis). Key analyses on the composition are summarized as below:

Supplementary Note 2 Supplementary
(1) For NaClO4-DME, the composition of the SEI is very sensitive to temperature.
Specifically, the NaCl (198.4 eV) component is ubiquitous at +20°C. In contrast, NaClO4 (208.5 eV) and NaClO3 (206.3 eV) are exhibited on the surface and NaClO2 (203.8 eV) is present at the inner zone at −20°C. Note that the presence of ClO4is possibly due to the precipitation/residual of the salt on the surface of Na metal. The detection of ClO3is possibly due to the decomposition of NaClO4. Such differences suggest that a lower temperature can alleviate severe corrosion caused by the reduction from NaClO4 to NaCl.